A communication system of the above kind is known from the handbook “Mobile Antenna Systems Handbook”, K. Fujimoto et al., Artech House, Inc., 1994, pp. 436-451. The known system is a land mobile satellite communication system in which the primary radio stations are satellites and the secondary radio stations are mobile radio stations in vehicles. The secondary radio stations comprise a phased array antenna system as a controllable structure. The phased array antenna system has adopted an open-loop tracking method with the hybrid use of a geomagnetic sensor and an optical-fiber gyro. In the present open-loop method the optical-fiber gyro is mainly used to give the information of vehicle movements, and the geomagnetic sensor gives an absolute direction to calibrate the accumulative error of the optical-fiber gyro at an appropriate time interval.